1. Field of the Invention
The present invention relates to an exhaust gas emission control system for an internal combustion engine, which is designed to increase further the exhaust gas emission controlling capability of a catalytic converter irrespective of running conditions of an engine or deterioration of the catalytic converter.
2. Description of the Related Art
A catalytic converter such as a three-way catalytic converter is provided in an exhaust passage of an internal combustion engine mounted on a vehicle for the purpose of removing unwanted exhaust gases such as hydrocarbons (HC), carbon monoxides (CO), oxides of nitrogen (NOx) and the like which are expelled from the internal combustion engine. Normally, in the catalytic converter of this type, noble metals are carried on a substrate, and oxidation reaction and reduction reaction of those substances of HC, CO and NOx are promoted by the noble metals.
The three-way catalytic converter has a function to adsorb oxygen to promote the reduction of NOx in a lean atmosphere (oxidation atmosphere), and then to desorb oxygen to promote oxidation  reactions of HC and CO in a rich atmosphere (oxidation atmosphere).
Incidentally, in recent years, from the viewpoint of reduced fuel consumption and protection of a catalytic converter, a so-called fuel cut is implemented in which a fuel supply is stopped when the vehicle is decelerated. However, when such a fuel cut is implemented, a large amount of oxygen comes to be contained in exhaust gases, and since the catalytic converter is saturated with adsorbed oxygen, when the fuel supply is resumed (fueling resumption), the removal of NOx gets deteriorated.
In order to suppress the increase in volume of NOx produced in association with such a fuel cut, fuel is controlled so as to be increased in volume to change the air-fuel ratio to a slightly richer air-fuel ratio than the stoichiometric air-fuel ratio (a rich change control). By this rich change control, unburned HC and CO, which are reducing agents, are caused to exist much in exhaust gases, so that unburned HC and CO are allowed to react with oxygen adsorbed in the catalytic converter for the adsorbed oxygen to be purged therefrom (oxygen purging). As this occurs, by setting appropriately a timing at which the rich change control ends, the fuel supply amount is normalized and the discharge amount of HC, CO and NOx can be suppressed to a maximum level.
As a technique of setting appropriately the rich change control ending timing, there has been known a technique in which the rich change control is ended when a sensor value  corresponding to a rich air-fuel ratio is outputted using an output of an air-fuel ratio sensor at an exit of a catalytic converter as a trigger. However, when the output of the air-fuel ratio sensor at the exit of the catalytic converter is used as the trigger, the rich change control is ended after oxygen adsorbed in the catalytic converter is completely desorbed therefrom (oxygen is completely purged) and exhaust gases, of which the air-fuel ratio is changed to the rich air-fuel ratio, start to be discharged. Due to this, fuel is supplied excessively, whereby unburned HC, CO are discharged, resulting in deterioration of the exhaust gas emission controlling capability of the catalytic converter.
As another technique of appropriately setting the rich change control ending timing, there has been proposed a technique in which a rich change control is set in advance to occur over a prescribed time period depending upon running conditions of the internal combustion engine, so that the rich change control is implemented over the prescribed time period so set, as disclosed in Japanese Patent Publication No. 2006-118433A. By implementing the rich change control over the prescribed time period which has been set in advance, the normalization of fuel supply amount according to the running conditions of the internal combustion engine can be realized when the fuel supply is resumed, thereby making it possible to  increase the NOx removing capability.
In recent years, there has been an increasing demand for a further improvement in exhaust gas emission controlling capability of catalytic converters. For example, there exists a fear that the rich change control ending timing cannot stay appropriate due to difference in oxygen adsorbing capability between individual catalysts, changing oxygen adsorbing capability due to aging and changing running condition of the internal combustion engine, and hence, an exhaust gas emission control system has been desired which can maintain a high exhaust gas emission controlling capability not only in an initial stage but also in those circumstances of aging (deterioration) of the catalytic converter and changing running conditions of the internal combustion engine by properly coping with changes caused in those circumstances.